Steel clad stainless composite article

ABSTRACT

This disclosure relates to a metallic composite article, i.e. ingot, slab, strip or sheet, having a thin stainless steel core sandwiched between two relatively thick low carbon or low alloy steel outer layers. The composite article is formed at the ingot stage by casting low carbon steel or low alloy steel simultaneously about a stainless steel plate suspended in a mold, which plate has had its major surfaces covered by a metallic protective layer so as to control the alloy depletion therefrom, especially the chromium to a value not less than about 11%, by weight. The resulting article exhibits excellent resistance to pitting and penetration in those corrosive environments where such phenomena are known to be a problem.

United States Patent 1191 Higbee et a1. 5] Sept. 9, 1975 [54] TEEL CLAD STAINLESS C MPOSITE 3,537,828 11/1970 Henrickson 29/196.1

ARTICLE 3,615,278 10/1971 Yamamoto 29/1961 3,623,901 11/1971 Forstmann 29/196.l Inventors: David Higbee; J p J p 3,775,194 11 1973 Dromsky 148 123 x both Of Middletown, Ohio 3,795,971 3/1974 Chivinsky 148/12 R [73] Assignee: Armco Steel Corporation,

Middletown Ohio Primary ExaminerL. Dewayne Rutledge Assistant Examiner-Arthur J. Steiner Filed: 8, 1973 Attorney, Agent, or FirmMelville, Strasser, Foster & 21 Appl. No.2 413,849 Hoffman Related US. Application Data [60] Continuation-impart of Ser. No. 162,660, July 14, [57] ABSTRACT 1971, abandoned, which is a division of Ser. No. This disclosure relates to a metallic composite article, 1969, Pat 3,621,561- i.e. ingot, slab, strip or sheet, having a thin stainless steel core sandwiched between two relatively thick [52] US. Cl. 9/1 low carbon or low alloy steel outer layers. The com- 196-6 posite article is formed at the ingot stage by casting [5 Int. low carbon steel or low alloy teel imultaneously [58] Fi ld Of Search about a stainless steel plate suspended in a mold, 29/1915 which plate has had its major surfaces covered by a metallic protective layer so as to control the alloy de- [56] References Cit d pletion therefrom, especially the chromium to a value UNITED STATES PATENTS not less than about 11%, by weight. The resulting arti- 1,306,690 6/1919 Gillespie 29/l96.1 C16 Q excellent resistance to pitting and Penetra- 1,896,411 2/1933 Maskrcy 29/l96.l tion In those Corrosive environments Where such p 2,219,352 10/1940 Andrus 29/196.1 nomena are known to be a problem. 3,326,647 6/1967 Holtzman 29/l96.l 3,393,445 7/1968 Ulam 29/196.1 x 14 Claims N0 Drawmgs STEEL CLAD STAINLESS COMPOSITE ARTICLE RELATED APPLICATION This application is a continuation of Ser. No. 162,660 filed July 14, 1971, now abandoned, which was a division of Ser. No. 796,616 filed Feb. 4, 1969, now US. Pat. No. 3,621,561, and entitled METHOD FOR FABRICATING A METALLIC COMPOSITE IN GOT.

BACKGROUND OF THE INVENTION There is hardly an industrial concern today that is not involved in the problems caused by corrosion. The annual cost of corrosion is estimated to run as high as $7,000,000,000.00.

Corrosion, which is the chemical breakdown of basic materials such as metal, is the result of environmental conditions. Where it is impossible to affect changes in the environment, the solutions to overcome the problems of corrosion necessarily shift to a consideration of the materials. And, while one obvious answer would be to select materials unaffected by the service to which the material is to be used, practical considerations often prohibit such a move. Therefore, the general approach by industrial concerns has been in the direction of selecting materials which offer the best resistance to corrosion at the lowest ultimate cost.

To select the best materials requires a study of the service conditions; this includes, among other items, the temperature changes, moisture content, stresses encountered, the nature of the corrosive action, etc.

The present invention was prompted by the need to find a material that would resist those corrosive conditions which promote localized attack commonly referred to as pitting. For example, one problem area was in the field of culverts and underground storage containers, which encounter such conditions as salt water marsh, fresh water marsh, mine water, alkali and acid soil and water, farm yield drainage, and domestic sanitary sewage. For the material to perform adequately under such severe conditions, it has to maintain a high level of strength, as well as resist pitting and therefore penetration from without. A corollary problem exists in automotive mufflers where condensate will collect and cause perforations and early failure. There are various other applications where general attack is not too severe, but where localized attack is a hazard such as in underground transformer tanks.

It has been demonstrated by service tests that in utilizing a composite article having a stainless steel core sandwiched between two layers of low carbon or low alloy steel, superior resistance to perforation was noted over a variety of bare and coated steels. For example, service corrosion tests on composite articles of this type were conducted by exposing test culverts in comparison with culverts of uncoated ingot iron, low alloy steels and other coated grades of steel. All of the culverts were buried in 21 installations representing seven classifications of corrosive environments normally encountered in culvert service. Periodic inspection over an extended period showed the composite article still exhibited good resistance to perforation, while nearly all of the remaining test culverts were corroded and almost destroyed, or severely perforated.

SUMMARY OF THE INVENTION Briefly, in the practice of this invention, a stainless steel plate whose thickness is at least about 0.5%, but preferably about 2-8%, of the total ingot, is secured vertically, in sandwich relationship between chill plates made of low carbon steel, through the center or other desired position of a mold. Only simple preparation is required for the surface of the stainless steel and chill plates, such as pickling or shot blasting.

Into the two cavities formed by positioning the stainless steel-low carbon steel composite in the mold, molten low carbon or low alloy steel is poured. After the solidification of the molten steel, the composite ingot is stripped and processed by the conventional steps which may include: surface conditioning, heating, hot rolling, cold rolling, shearing, and coiling.

The foregoing procedure results in a composite structure which exhibits an excellent bond between the stainless steel and the low carbon or low alloy steel. While the starting ratios between the two materials and the percent reduction thereafter will generally dictate the final thickness of the layers, for practical purposes, the stainless steel layer should be no less than 0.001 inches in the finished product. Preferably, it should be at least 0.005 inches thick to insure that the core is continuous in the final product, as well as providing a product which resists mechanical damage.

From the foregoing, it should be evident that the composite article of this invention comprises essentially three layers, which layers vary in metallurgical characteristics from the outside cast layers toward the inside stainless steel core. Of course if the chill plates are of the same material as the outer layers, the demarcation between these layers disappears. Specifically, there is a variation of chemistry, crystalline structure and grain size in the outer layers from the outside to the said core, which variation results in part from the use of metallic protective layers adjacent the stainless steel core. The ratio of the total thickness to the stainless steel core thickness of a final hot rolled product is about 10:1 to about 50:1. Lower ratios, on the order of 3:1, may be developed from the same starting product by further cold reductions. This phenomenon occurs since the stainless steel core is reduced less than the outer low carbon or low alloy steel layers.

DETAILED DESCRIPTION It was indicated previously that this invention contemplates the provision of a metallic composite article characterized by a thin, stainless steel core sandwiched between two relatively thick low carbon or low alloy steel layers. Such composite article is particularly valuable in moist environments where pitting-type corrosion is a primary problem. The term low carbon steel refers to a steel having less than 0.15% C (Metals Handbook, 8th Edition, Vol. 1, 1961, page 79).

At this juncture, it may be helpful to consider the phenomenon of corrosion and the manner, at least in theory, by which the present invention meets the problem. Generally considered, the corrosion of metals is an electrochemical occurrence. That is, the flow of electric current is associated with the phenomenon. From this it would follow that excessive amounts of moisture, which is the vehicle for the current flow, tend to accelerate the action. From the above, it can be analogized that the phenomenon of corrosion is like that of a battery or electrolytic cell. In the respective systems, there 3 is an anode and a cathode in a conducting solution, wherein said solution is called the electrolyte. Therefore, one of the major factors in the rate of corrosion is the magnitude of the current flow in the electrolyte. By directing said current flow, or in some way controlling it, the rate of corrosion can be minimized.

One of the foremost methods used by the prior art has been in the selection of an anodic or sacrificial coating on the metal to be protected. Hot dipped zinc coated steel is a prime example of the latter method. In this method, a steel core is provided with a layer of zinc by the continuous hot dip or galvanizing process. In service, the zinc acts like a barrier film to the steel core. Once the zinc layer is penetrated by the action of the corrosive environment, galvanic protection takes over. This galvanic protection just described is essentially the same as found with the present invention. However, in the former situation, once broad areas of the steel core become exposed, general corrosion of the core takes place. In the latter, protection continues despite the exposure of the core.

Thus, as indicated above, the composite article, as taught herein, contemplates a cathodic stainless steel core in a matrix of anodic low carbon or low alloy steel. Since the stainless steel is cathodic to, or less anodic than, the mild steel in corrosive environments, the corrosion of the mild steel protects the inner layer of stainless steel. However, when a corroded area or pit reaches the inner core of stainless steel, the pit enlarges rather than a penetration of the core. This was different than the combination of zinc on steel since the general corrosion rate of a plain carbon steel core is considerably higher than the rate of a stainless steel core.

A further development recognized in the composite article of the invention is the synergistic relationship between the stainless steel core and the outer layers of low carbon or low alloy steel under conditions of localized corrosion. That is, it has been found that under severe pitting conditions the performance of the composite was superior to that of equal thickness stainless steel. While there is no desire to be bound by any theory, it is believed that as the corrosion proceeds, the phenomenon of polarization takes place. Under this theory, it is believed that hydrogen is deposited on the surface of the cathodic stainless steel, thereby retarding the current flow which tends to lower the rate of corrosion. It will be acknowledged that this result is apparent only in cases of localized corrosion or pitting such as occurs under chloride attack. Under conditions of generalized corrosion, a stainless steel plate alone would offer better protection.

It should be apparent from the foregoing that an economical product has been developed which has excellent resistance to pitting type corrosion. This is accomplished by utilizing a composite article having a stainless steel core in the ratio of about 1:3 to about 1:50 of the total thickness for the three-layer composite. The core should be at least 0.001 inches thick, but preferably 0.005 inches thick to insure core continuity.

Further attributes of this invention adding to its economical character are the material costs, and the method of producing, which method will now be described. Upon inspection of the product of this invention, several methods may appear plausible to those skilled in the art. None of them, however, approach the metallurgical soundness nor the economics realized herein.

The preferred procedure forms the composite article at the ingot stage, an early step in producing steel. A stainless steel plate, whose thickness when compared with the mold falls within the range described above, is disposed along the axial center of, and parallel to, two sides of the mold. However, it should be understood that said stainless steel plate may be positioned off-cen ter of the mold. In either case, to each major surface of said plate there is provided a protective layer or film. This layer is provided to inhibit alloy diffusion between the outer layers and the stainless steel plate, as well as to substantially prevent mechanical mixing between the cast outer layers and the stainless steel core. Two particular elements to be inhibited from diffusion are chromium and carbon. In the preferred embodiment the protective layer will be in the form of low carbon steel chill plates placed adjacent the stainless steel core. This arrangement results in substantially two identical cavities in the preferred embodiment, on either side of the plate. Into the top of each said cavity a quantity of molten low carbon or low alloy steel is poured to fill the respective cavities. It will be understood that this may be a single or double nozzle pouring. The casting may be accomplished by means of a tundish having dual nozzles. This procedure requires a minimum of surface preparation for the stainless steel insert plate. Generally, a minor pickling or shotblasting step may be used. With other known methods, considerable surface preparation is required.

Depending on the ability of the protective layer to inhibit depletion, a dramatic change will be detectable on the composition variation from side-to-side. As an incident of this development, experiments were conducted on the effect of casting low carbon steel about stainless steel inserts without the use of a protective layer. As a result of the casting and solidification phenomenon taking place in the mold, a severe alloy depletion of the core occurred. While a central core area was discernible, it had a uniform but considerably lowered alloy analysis over that originally used. The chromium content was reduced by as much as 72%.

While not desiring to be bound by any theory regarding the severe core alloy depletion, it is believed that during the casting operation a skin of low carbon or low alloy steel solidifies on the core plate. Since a stainless steel core [for example, AISI Type 304] has a melting point below that of the low carbon or low alloy steel, there is sufficient heat to melt the stainless steel core. The solidified skin thickens and the alloy constituents of the stainless steel core begin to diffuse into the adjacent skin. It is believed that this lowers the melting point of a portion of the adjacent skin and that portion is remelted. Mixing ensues and a core of reduced alloy content forms. The new depleted core can be twice or more its initial volume when it finally stabilizes and solidifies. This procedure results in a composite structure, but one having a low carbon or low alloy steel outer skin and a non-stainless steel core.

It was discovered by the present invention that with the addition of a protective layer, such as low carbon or low alloy chill plates, it was possible to confine the core considerably such that only a minor alloy depletion resulted. For instance, in one series of examples where the chill plates varied between about 2.5 percent to 0.625 percent of the ingot thickness, the corresponding chromium depletion ranged from about 0.4% to about 29%. Even in the most severe case, the core was still considered stainless steel, i.e., at least 11% chromium, by weight, remained in the core.

Continuing now with the processing of the solidified composite ingot, a standard rolling schedule-was followed to reduce the ingot to a slab, then to a strip. While the processing subsequent to the casting of the ingot forms no part of this invention, such processing might includei 1. Heating to about 2350 F.,

2. Hot rolling to a slab thickness of about 7 inches,

3. Surface conditioning such as by scarfing, 4. Heating to about 2380 F., 5. Hot rolling to a strip thickness of about 0.180 inches,

6. Pickling to remove scale, and

7. Cold rolling to desired gauge.

For the purpose of providing those skilled in the art with greater insight into this invention, an example is presented as illustrative of this invention.

Prior to casting the ingot, 0.5 inch thick low carbon or low alloy steel plates were tack welded to the major surfaces of a Type 304 (Austenitic) stainless steel slab which measured about 46 X 78 X 1.0 inches. To assist in preventing the core plates from floating up during the casting operation, two 2.5 inch legs were welded onto the composite and attached to a base plate. The entire assembly was then placed on a mold stool and a 23 X 50 X 84 inch ingot mold was set in position. After centrally positioning the composite, exothermic side boards were placed inside the top of the mold to control the shrinkage in accordance with standard steel making practice. Aluminum killed molten steel, whose chemistry was as follows:

.l 1% .0037! .balance carbon manganese sulfur phosphorus copper tin iron

TABLE I plated, a molybdenum bearing austinitic stainless steel may be desired. On the otherhand, a Type 410 stainless steel will be adequate under less severe conditions. Therefore, stainless steel as contemplated herein includes all ferrous alloys containing about 1 1% or more chromium, and other alloying elements normally found therein.

-A further and final feature contemplated by this invention is the provision of an exterior coating on the finished clad product. It may be desirable to secure additional general corrosion protection or merely to en hance the appearance of the product. Accordingly, the coating may be metallic such as zinc, aluminum, terne, lead, chromium, nickel, cadmium, etc., or a non-metallic such as asphalt, paint, plastics, etc. However, this listing is merely illustrative and should not be read as excluding others. No attempt will be made here to sug gest a coating or combination of coatings for a given environment as it is believed that a skilled worker in the art will know the most appropriate coating to be used, and the manner of applying same.

In view of the variations which may become apparent to those skilled in the art, no limitation is intended to be imposed herein except as set forth in the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A flat metallic composite cast ingot comprising a thin stainless steel core whose thickness is at least 0.5% of said article fusion bonded between two thicker outer layers of steel selected from the group consisting of low alloy steel and low carbon steel, the said stainless steel core exhibiting substantially its original integrity, by virtue of minimum diffusion of carbon into the stainless steel, and minimum diffusion of chromium out of the stainless steel.

2. A composite article in sheet or strip form, produced from an ingot according to claim 1, wherein said stainless steel core is at least 0.001 inches.

3. The composite article according to claim 2, wherein the ratio of said article thickness to said core thickness is between about 3:1 to :1.

4. The composite article according to claim 3, wherein the ratio of the article thickness to the core thickness is at least 10:1.

5. The composite article according to claim 2, wherein the core thickness is at least 0.005 inches.

TYPICAL MECHANICAL PROPERTIES Yield Strength PS1 Tensile Strength PSI Direction Elong.

Hardness Longitudinal Transverse 6. The metallic composite article according to claim 1, wherein each of said outer layers is characterized by a variation in metallurgical characteristics from the outside to the stainless core.

7. The metallic composite article according to claim 6, wherein each of said layers is characterized by a variation in chemistry, crystalline structure and grain size from the outside to the stainless core.

8. A flat metallic composite cast ingot comprising a thin austenitic stainless steel core whose thickness is at least 0.5% of said article fusion bonded between two 7 thicker outer layers of steel selected from the group consisting of low alloy steel and low carbon steel, said stainless steel core exhibiting substantially its original integrity, by virtue of minimum diffusion of carbon into the stainless steel, and minimum diffusion of chromium out of the stainless steel.

9. A composite article in sheet or strip form, produced from an ingot according to claim 8, wherein said stainless steel core is at least 0.00] inches.

10. The composite article according to claim 9, wherein the ratio of said article thickness to said core thickness is between about 3:1 to 50:1.

size from the outside to the stainless steel core 

1. A FLAT METALLIC COMPOSITE CAST INGOT COMPRISING A THIN STANLESS STEEL CORE WHOSE THICKNESS IS AT LEAST 0.5% OF SAID ARTICLE FUSION BONDED BETWEEN TWO THICKER OUTER LAYERS OF STEEL SELECTED FROM THE GROUP CONSISTING OF LOW ALLOY STEEL AND LOW CARBON STEEL, THE SAID STAINLESS STEEL CORE EXHIBITING SUBSTANTIALLY ITS ORIGINAL INTEGRITY, BY VIRTUE OF MINIMUM DIFFUSION OF CARBON INTO THE STANILESS STEEL, AND MINIMUM DIFFUSION OF CHROMIUM OUT OF THE STAINLESS STEEL.
 2. A composite article in sheet or strip form, produced from an ingot according to claim 1, wherein said stainless steel core is at least 0.001 inches.
 3. The composite article according to claim 2, wherein the ratio of said article thickness to said core thickness is between about 3:1 to 50:1.
 4. The composite article according to claim 3, wherein the ratio of the article thickness to the core thickness is at least 10:1.
 5. The composite article according to claim 2, wherein the core thickness is at least 0.005 inches.
 6. The metallic composite article according to claim 1, wherein each of said outer layers is characterized by a variation in metallurgical characteristics from the outside to the stainless core.
 7. The metallic composite article according to claim 6, wherein each of said layers is characterized by a variation in chemistry, crystalline structure and grain size from the outside to the stainless core.
 8. A flat metallic composite cast ingot comprising a thin austenitic stainless steel core whose thickness is at least 0.5% of said article fusion bonded between two thicker outer layers of steel selected from the group consisting of low alloy steel and low carbon steel, said stainless steel core exhibiting substantially its original integrity, by virtue of minimum diffusion of carbon into the stainless steel, and minimum diffusion of chromium out of the stainless steel.
 9. A composite article in sheet or strip form, produced from an ingot according to claim 8, wherein said stainless steel core is at least 0.001 inches.
 10. The composite article according to claim 9, wherein the ratio of said article thickness to said core thickness is between about 3:1 to 50:1.
 11. The composite article according to claim 10, wherein the ratio of the article thickness to the core thickness is at least 10:1.
 12. The composite article according to claim 9, wherein the core thickness is at least .005 inches.
 13. The metallic composite article according to claim 8, wherein each of said outer layers is characterized by a variation in metallurgical characteristics from the outside to the stainless steel core.
 14. The metallic composite article according to claim 13, wherein each of said layers is characterized by a variation in chemistry, crystalline structure and grain size from the outside to the stainless steel core. 